In a variety of electronic devices, metal conductive strips are embedded in, glued to, or enclosed between two dielectric sheets. These devices may be simple printed circuit boards, or more sophisticated microwave stripline phase adjusters, direction couplers or the like. In these sophisticated devices, the dielectric sheet assumes an active role in the device and aside from providing support, the dielectric material must transmit signal pulses. This function is referred to as stripline transmission and the dielectric material used for this function is termed herein, "stripline board." Discussions of devices which may employ a variety of stripline board materials are found in U.S. Pat. No. 3,513,414 and U.S. Pat. No. 3,575,674.
A wide variety of dielectric material has been used as stripline board, for example, polystyrene, polyethylene, and other polyolefins. These polymers are used when, as is often the case, material with a low dielectric constant (3 or less) is required. This material must have a uniform dielectric constant within each sheet and the dielectric constant should not vary from sheet to sheet. The dielectric constant should be independent of frequency over the band width need for a particular application. The material should not dissipate excessive energy, i.e., it should have a low loss tangent, below 0.0018 at 10 GHz. The mechanical stability of the dielectric material is also important. Shrink, warpage, thermal expansion and cold flow should be minimized.
Unfortunately, presently used polymeric stripline boards have unsatisfactory mechanical and/or dielectric properties, especially in the high frequency area of 10 GHz and above.
In their search for improved dielectric materials, the inventors turned to glass sphere reinforced plastic composites. Glass beads or microspheres have been used for many years, but their use as a filler/reinforcement in plastics is relatively recent. They are commonly used to reduce costs and improve mechanical properties of structural plastics in the form of so-called "syntactic foams" in which the glass beads are physically held in suspension by the plastic binder or matrix. Glass beads as used in these composites are generally considered to be chemically inert. A good discussion of glass beads in plastic composites is given by James Ritter in Polyblends and Composites, ed. by Paul F. Bruins, Applied Polymer Symposium No. 15 (Interscience Publishers 1970).